WASHINGTON (AP) — The U.S. economy grew at a solid 6.4% rate in the first three months of this year, setting the stage for what economists are forecasting could be the strongest year for the economy in possibly seven decades.

The Commerce Department said Thursday that growth in the gross domestic product, the country's total output of goods and services, was unchanged from two previous estimates. The gain represented an acceleration from growth at a 4.3% rate in the fourth quarter.

Economists believe GDP growth has accelerated even more in the current April-June quarter as increased vaccinatio...